The present invention relates to a hybrid integrated-type planer lightwave circuit platform to be used for optical transmissions, and its method for manufacturing.
As a method to improve mass-productivity of an optical module to be used in the field of optical transmissions, and reduce the cost thereof, the mounting method by which passive parts such as an optical coupler and active parts such as a light receiving/emitting element are hybrid-integrated has been widely noticed. Particularly, the method for mounting active parts on a platform having planar lightwave circuits (PLC) has been noticed in terms of its excellent mass-productivity and the integration of various optical circuits and a light receiving/emitting element enabled by the method.
A prior-art hybrid integrated-type planar lightwave circuit platform has the structure shown in FIG. 4, for example, and the PLC platform has a silica-based waveguide formed on a Si substrate, and is manufactured by the following processes. That is, in FIG. 4:
1) Uneven steps having a predetermined planar patterns are formed on the Si substrate 1 to form a terrace portion 1a. This terrace portion 1a is loaded with optical elements later.
2) On the Si substrate 1, silica-based soot is deposited by means of flame hydrolysis deposition method (FHD), and transparent-vitrified at a high temperature, whereby a lower cladding layer 2 made from silica-based glass is formed. As gases for the material of the silica-based soot, SiCl4 to become silica glass SiO2 and PCl3 and BCl3 to become dopants P2O5 and B2O3 for adjusting the glass softening temperature are used. P2O5 and B2O3 are dopants having the effect of lowering the glass softening temperature of silica glass. The sintering temperature for vitrification to change the soot condition into the transparent glass condition is set to be higher than the glass softening temperature of the lower cladding layer 2.
3) Then, the surface of the covered Si substrate 1 is mechanically ground to expose the terrace portion 1a and make the surface flat.
4) Next, height adjusting layer 3 made from silica-based glass with the same refractive index as that of the lower cladding layer 2 is formed. Dopants P2O5 and B2O3 which have the effect of lowering the glass softening temperature are added to the height adjusting layer 3. The amount of dopants to add is four times the amount of dopants added to the lower cladding layer 2. Furthermore, the temperature for transparent-vitrification of the height adjusting layer 3 is set to be higher than the glass softening temperature of the height adjusting layer 3. This height adjusting layer 3 is provided in order to adjust activation layer 6a of optical element 6 to be loaded later and core layer 4 of the waveguide to be equal in height.
5) Then, the core layer 4 made from silica-based glass is formed. To the core layer 4, dopants P2O5 and B2O3, which have the function of lowering the glass softening temperature of quartz glass, are added by an amount being approximately 1.1 times of the amount added to the lower cladding layer 2, and furthermore, GeO2 is added so that the refractive index of the core layer becomes approximately 0.5% greater than those of the lower cladding layer 2 and height adjusting layer 3. The temperature for transparent-vitrification of the core layer 4 is set to be higher than the glass softening temperature for the core layer 4.
6) Next, the core layer 4 is etched by means of reactive ion etching (RIE) so as to have core patterns having a predetermined function.
7) Next, upper cladding layer 5 made from silica-based glass is deposited, and the core layer 4 is buried therein. To the upper cladding layer 5, dopants P2O5 and B2O3 which have the effect of lowering the glass softening temperature are added by an amount being approximately three times the amount of dopants added to the lower cladding layer 2. In addition, the temperature for transparent-vitrification of the upper cladding layer 5 is higher than the glass softening temperature thereof, and the upper cladding layer is transparent-vitrified at a temperature lower than the glass softening temperature of the core layer 4.
8) The height adjusting layer 3, core layer 4, and upper cladding layer 5 on the terrace portion 1a are removed.
9) A predetermined pattern of electrode 7 is formed on the terrace portion 1a. 
The height adjusting layer 3 is used to make the activation layer 6a of the optical element 6 loaded on the terrace portion 1a and core layer 4 to be equal in height, and is a thin-film with a thickness of 10 xcexcm or less.
In the case where a glass film is formed by means of FHD, the silica-based glass film has properties whereby if the silica-based glass film is thin, transparent-vitrification by means of sintering becomes difficult, and also, if the amount of added dopants P2O5 and B2O3 increases, transparent-vitrification becomes easier.
Therefore, since the height adjusting layer 3 is thin (thickness: 10 xcexcm or less), in order to make transparent-vitrification thereof easier, dopants are added to the height adjusting layer 3 by a larger amount than that of dopants added to the core layer 4 and upper cladding layer 5.
The quartz glass has a property whereby if the dopant amount is increased, the glass softening temperature lowers. Therefore, when the glass softening temperatures of the lower cladding layer 2, height adjusting layer 3, core layer 4, and upper cladding layer 5 are TU, TT, TC, and TO, the relationship among them is as shown in expression (1). The glass softening temperature of the height adjusting layer 3 is lower than that of the core layer 4 and upper cladding layer 5.
TU greater than TC greater than TO greater than TTxe2x80x83xe2x80x83(1)
Since the temperatures for transparent-vitrification of the core layer 4 and upper cladding layer 5 are higher than the glass softening temperatures TC and TO of the core layer 4 and upper cladding layer 5, when sintering and transparent-vitrifying the core layer 4 and upper cladding layer 5, the height adjusting layer 3 whose glass softening temperature is lower than that of the core layer 4 and upper cladding layer 5 is softened, and the core pattern of the core layer 4 formed on the height adjusting layer is deformed, and therefore, it becomes difficult to stably obtain desired optical performance.
The present invention is made in order to solve the abovementioned problems, and the object of the invention is to provide a planar lightwave circuit platform constructed by forming a silica-based lower cladding layer, height adjusting layer, core layer, and upper cladding layer in order on a substrate, wherein the glass softening temperature of the height adjusting layer is higher than that of the core layer and upper cladding layer.
Herein, the height adjusting layer is provided in order to adjust the activation layer of a loaded optical element and core layer to be equal in height.
In another aspect of the planar lightwave circuit platform of the invention, the temperature for transparent-vitrification of each layer is higher than the glass softening temperature of the same layer, and the glass softening temperature of the height adjusting layer is higher than the temperatures for transparent-vitrification of the core layer and upper cladding layer.
The planar lightwave circuit platform of the invention is used for an optical module as one of purposes for use.
Another object of the invention is to provide a method for manufacturing a planar lightwave circuit platform which has a process to form a silica-based lower cladding layer, height adjusting layer, core layer, and upper cladding layer in order on a substrate, wherein silica-based soot having a sufficient thickness for transparent-vitrification by means of sintering is deposited by means of FHD, and transparent-vitrified, and then formed in a predetermined thickness, whereby the height adjusting layer is thinly formed.
Herein, the height adjusting layer is provided in order to adjust the activation layer a loaded optical element and core layer of to be equal in height. Furthermore, the silica-based soot having a sufficient thickness means a silica-based soot which has a thickness to be transparent-vitrified by means of sintering in the condition where the soot is added with dopants by a predetermined amount.
In the method for manufacturing a planar lightwave circuit platform of the invention, the lower cladding layer, height adjusting layer, core layer, and upper cladding layer is formed through transparent-vitrification treatment, and preferably, the temperature for transparent-vitrification of each layer is set to be higher than the glass softening temperature of the same layer, dopants are added to each layer to lower the glass softening temperature, and the amount of dopants to be added to the core layer and upper cladding layer is increased more than that of dopants to be added to the height adjusting layer to make the glass softening temperature of the height adjusting layer higher than the temperatures for transparent-vitrification of the core layer and upper cladding layer.
After transparent-vitrification of the height adjusting layer, etching or grinding is preferably employed for formation in a predetermined thin thickness.
According to the invention, the glass softening temperature of the height adjusting layer is adjusted to be higher than that of the core layer and upper cladding layer. Therefore, when transparent-vitrifying the core layer and upper cladding layer at a high temperature, the temperature can be set so that the height adjusting layer is not softened. Accordingly, since the height adjusting layer is not softened, the core layer with a pattern is prevented from being deformed, and a waveguide circuit which is sensitively influenced by the pattern form of the core layer (for example, Mach-Zehnder interferometer) can be stably formed on the substrate. Furthermore, by applying the planar lightwave circuit platform of the invention to an optical module, an optical module which is excellent in optical performance and high in reliability can be provided.
Furthermore, according to the invention, in order to form the height adjusting layer, since silica-based soot is formed in a sufficient thickness so as to be transparent-vitrified by means of sintering, and then a glass film is formed in a predetermined thin thickness, satisfactory transparent-vitrification is achieved even when the softening temperature is high, that is, a composition in which the amount of dopants is less is employed. In addition, when transparent-vitrifying the core layer and upper cladding layer at a high temperature, the temperature can be set so that the height adjusting layer is not softened, and since the height adjusting layer is not softened, a waveguide circuit can be stably formed on the substrate.
Moreover, by reducing the thickness of the height adjusting layer by means of etching and grinding, it can be precisely formed in a required and predetermined thickness.
As mentioned above, the invention has an excellent effect whereby optical performance of the planar lightwave circuit on the planar lightwave circuit platform is stabilized.